Cardiac arrhythmias affect millions of people in the United States. Many are prescribed medication, but more severe cases are treated with RF ablation of myocardial (heart muscle) tissue, administered by catheterization. Ablation targets are typically chosen using electro-anatomical (EA) mapping, where a sensing catheter is placed at many locations in the heart chamber to analyze the electrograms and determine where reentrant circuits may be occurring. Using this information, an ablation catheter is then navigated to those targets, using x-ray fluoroscopy and the EA map. However, since myocardium is all but invisible in x-ray, targeting inaccuracies are frequent. It is also impossible to tell with any certainty which areas have been ablated (the lesions), and to what extent, or if any gaps still exist. As a result, recurrence of arrhythmias after ablation therapy is quite common, requiring repeated treatment. These shortcomings can be addressed with magnetic resonance imaging (MRI). With MRI, myocardium is readily visible, easily distinguishable from blood, and sufficient detail is available to recognize anatomical landmarks, al, of which can improve targeting accuracy. Typical MR imaging methods for visualizing ablation lesions employ a T1Cshortening gadolinium-based contrast agent (GBCA) to identify damaged or dead tissue. In healthy myocardium, contrast agent washes in then washes out over a few minutes. In necrotic myocardium, the wash out process is compromised, so the contrast agent remains in the tissue and is enhanced in T1Cweighted MR images, referred to as late gadolinium enhancement (LGE). However, the need for a contrast agent in current methods is a disadvantage for multiple reasons. There are complexities in image interpretation and dosage limitations, limiting utility in monitoring treatment. For visualizing ablation lesions acutely, we have developed an efficient MR imaging technique which does not require the use of contrast agent. This method, which we hope to develop, optimize and test in this proposal, shows enhancement in the necrotic (dead) region of the lesion, with edema seen as a dark band around the enhancement. Our preliminary results show reliable and readily seen lesions in both thick and thin myocardial walls. This could be of great benefit before (screening, planning), during (targeting, assessing) or after (evaluating) an ablation procedure. In obtaining our preliminary results, we have chosen imaging parameters which produce readily visible contrast between ablation lesions, normal myocardium and cavitary blood. We wish to improve and validate the technique, optimizing imaging parameters and protocols, study the results, and investigate how this may be used in clinical application. Lesion images obtained with the proposed technique may be used to guide an ablation procedure whether performed under guidance of x-ray/ultrasound (typical) or MRI (under investigation). In either case, this technique can be used to evaluate the treatment, by answering the question: Have all target regions of myocardium been ablated to the point of necrosis, which is known to cause lasting conduction block? Images such as these will help to determine whether the procedure is finished, or if further ablations are required to complete coverage. Future study can follow patients to evaluate the impact on recurrence.